The present invention relates generally to signal processing and, more particularly, to a method and an apparatus for motion compensation of video images.
Motion compensation of video images refers to estimating and compensating the relative motion between adjacent video frames and is used as a method facilitating data compression or as a method for mitigating movement errors during conversion processes such as line number conversion or telecine. An encoder employing motion compensation may, therefore, reduce redundant information by estimating the interframe motion and passing this information to the decoder, which is able to use information already received for temporally antecedent frames in order to form the current frame. Furthermore, motion compensation may also reduce the number of bits required to encode interframe data by decreasing the magnitude of the interframe difference.
Typically, motion estimation for a frame is performed by subdividing the image into a plurality of regions, such as 8.times.8 pixel blocks, and a motion vector is determined for each block based on comparisons with displaced versions of the previous frame. Interframe difference information is calculated according to the appropriate block displacements indicated by the motion vectors.
Image reconstruction occurs by using the motion vector for each block in conjunction with interframe difference information. Inherently, this method requires random access of image data since contiguous blocks in one frame may no longer be contiguous in the subsequent frame due to their relative motion and is, therefore, not conducive to memory organization which would expedite image reconstruction and thereby ease hardware requirements and free time for alternative processing.
Employing motion compensation for data reduction is known in the art and U.S. Pat. No. 5,006,929 describes such a system which estimates local motion vectors and a global motion vector for each frame and calculates the difference between the global motion vector and each local vector for each frame. This method is well suited for variable length coding for the difference data and is also immune to frame-to-frame accumulative error. Although variable length coding reduces the data, this attribute occurs at the expense of additional hardware and processing time for the motion compensation.
A system applicable to converting from a high definition television (HDTV) signal having 1125 lines, 60 fields to a PAL signal having 625 lines, 50 fields is described in U.S. Pat. No. 4,731,651. This system provides for improved accuracy for vertical movement compensation, with 1/2 line resolution accomplished essentially by first forming two PAL frames from alternative lines of each HDTV frame, and then forming the output PAL frame by merging the appropriate lines from the two PAL frames based on the motion estimation to form a resulting output PAL frame with 1/2 line vertical resolution. This system is not adaptable, however, to 1/2 line vertical resolution for systems which do not convert from a source signal with approximately twice the vertical line resolution as the resultant signal.
In digital video transmission systems, not only is motion compensation an essential function in the data reduction process but also, for practical implementation it must be performed within the data rate limited video processing time and within the hardware cost limitations inherent to a system where each receiver contains motion compensation circuitry associated with the video decoder.
Accordingly, an object of the present invention is to provide a cost effective method and system for video motion compensation which reduces the amount of encoded information for transmission.
A further object of the present invention is to provide a video motion compensation method and system with 1/2 pixel horizontal and 1/2 line vertical resolution.
The foregoing specific objects and advantages of the invention are illustrative of those which can be achieved by the present invention and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of the invention will be apparent from the description herein or can be learned from practicing the invention, both as embodied herein or as modified in view of any variations which may be apparent to those skilled in the art.